use javax.annotation.Nullable in S2AProtocolNegotiatorFactory.

Author: rmehta19

s2a/src/main/java/io/grpc/s2a/handshaker/S2AProtocolNegotiatorFactory.java

@@ -46,7 +46,7 @@
 import java.util.List;
 import java.util.Optional;
 import java.util.concurrent.Executors;
-import org.checkerframework.checker.nullness.qual.Nullable;
+import javax.annotation.Nullable;
 
 /** Factory for performing negotiation of a secure channel using the S2A. */
 @ThreadSafe